1. Field of the Invention
The present invention relates to a light transmitter and a peak compression circuit used in optical communication for modulating a light source such as a semiconductor laser and for transmitting a frequency division multiplexed signal or a multiplexed signal of spread spectrum signals. More particularly, the present invention relates to a method of preventing deterioration in performance due to nonlinearity of such a circuit or device.
2. Description of the Related Art
A semiconductor laser has input and output characteristics such that it does not output light when the current of a modulating signal input thereto is below a certain threshold. Therefore, the modulating signal current of a semiconductor laser which is a frequency division multiplexed signal or a multiplexed signal of spread spectrum signals is not converted into an output light signal where it falls below the threshold of the semiconductor laser. In other words, the parts of the modulating current below the threshold of the semiconductor laser are lost instead of being converted into an output light signal. This phenomenon is referred to as "clipping".
When a frequency division multiplexed signal or a multiplexed signal of spread spectrum signals is optically transmitted, transmission channels are normally set so that the optical modulation index of those channels will total at 100% or more. Therefore, there is a possibility of clipping which is a phenomenon that a modulating current momentarily falls below the threshold of a semiconductor laser, although the probability is low. Clipping results not only in loss of the information included in the portion wherein it has occurred but also in distortion which degrades the performance. In the case of a digital modulating signal, clipping creates another problem in that it generates digital errors. Further, the problem as described above will be encountered also in light modulators for modulating unmodulated light, LEDs and the like because they have a point having no linearity for the input and output characteristics thereof which is comparable to the threshold of a semi-conductor laser.
Such a phenomenon can occur also in electrical amplifiers wherein the input and output characteristics are linear in a limited portion and in other portions the characteristics are non-linear. The only difference between these two case is that the transition at the boundaries of such portions is steep for a semiconductor laser while the transition normally proceeds gradually in an electrical amplifier. Therefore, the problem as described above exists also in an electrical amplifier or the like where the input signal is susceptible to nonlinearity because its magnitude is large.
Such deterioration in performance is described in papers such as A. A. M. Saleh, "Fundamental Limit on Number of Channels in Subcarrier-Multiplexed Lightwave CATV System", Electron. Lett., Vol. 25, pp. 776-777, but there has been no report on how to reduce deterioration due to clipping.